The present invention relates generally to stator housings or frames for dynamoelectric machines, and more particularly to new and improved stator frames and methods of making same.
It is conventional in the design of many types of dynamoelectric machines, such as electric motors and generators, to support a stator core within a stator housing or frame. The stator housing or frame may have external support feet or pads which facilitate mounting on a suitable support or apparatus such as a machine tool in a predetermined orientation, and defines a cavity to receive a stator core. Suitable bearings and support structure are also provided to rotatably support a rotor in coaxial relation within a bore in the stator core. In motors and generators it is a common practice to provide generally longitudinal fins on the external surface of the stator housing or frame which enhance cooling by means of air passing over or around the fins during operation.
In general, cast stator frames, housings or shells of prior dynamoelectric machines have been made of cast iron by sand casting processes. The sand casting process generally entails burying a pattern with packed sand defining at least one parting plane, removing the pattern to form a mold cavity, positioning a separate core piece within the mold cavity to define the stator core accommodating cavity of the stator frame, positioning separate pieces that will define mounting feet or pads on the casting and, if desired, one or more conduit box support pads to finalize the mold cavity. Molten iron is then poured into the mold cavity. After solidifying and cooling, the casting is removed and cleaned, leaving a relatively rough surface casting. This process is very time consuming, generally taking several hours from start to finish.
A significant drawback in making stator housings by such sand casting processes is that the sand-cast stator housing must undergo substantial machining. For example, a common technique for mounting an annular stator core within the stator housing cavity is to cold-press the stator core into the bore. In this procedure, the as-cast interior of the housing generally requires significant machining to bring the dimensional configuration thereof to a proper size and tolerance range. An alternative procedure for mounting stator cores within stator housings is by known heat shrink techniques. In this procedure, the wall thickness of the stator frame must be relatively precisely machined to have uniform thickness walls to insure uniform and low stress shrinkage after heating the housing to receive the stator core. Sand-cast stator housings thus generally require substantial machining to prepare the as-cast housing for assembly with a stator core by heat shrink methods. Moreover, the heat shrink process generally takes two to three hours to complete. Also, the opposite end surfaces on the stator housing generally require significant machining to prepare them for mating relation with rotor shaft bearing support frames, commonly also referred to as end shields. Additional machining may be necessary when a cooling fan cover or shroud is to be supported on one end of the stator housing for directing fan-driven air over the cooling fins.
Thus, conventional sand-cast stator frames are associated with expensive and labor intensive machining operations and generally result in significant material waste, all of which adds to their cost of manufacture.
Another very significant drawback with sand-cast stator housings or shells is that a sand mold imposes substantial limitations on the stator housing design. For example, in larger size motors and generators where heat transfer, i.e. cooling, is a particularly important factor, sand casting characteristics limit the relative height and thickness dimensions of the cooling fins formed on the external surface of the stator housing. More specifically, the rough sand surfaces defining the fin cavities create a relatively high friction interface with the poured molten metal, causing the molten metal to flow slowly at the interface with the sand. If the fin height to fin thickness ratio is relatively high, as desired to obtain optimum cooling, the molten metal may solidify before it completely fills the fin cavity, thereby resulting in an incomplete fin or a non-uniform fin surface, either of which may result in a defective casting. Further, casting material has a tendency to crack and break the sand mold before the molten metal reaches the full depth of the fin cavities. This phenomenon results in disadvantages that practice of the present invention overcomes.
In an attempt to overcome the drawbacks associated with sand cast stator housings, alternative techniques have included consideration of making stator housings by lost foam casting processes. This type of technique or process, which may also be termed evaporative pattern or evaporative foam casting, generally entails making one or more metallic tools or intermediate molds which define a cavity substantially equal to the finished cast product desired, or a portion of the finished product. The intermediate mold cavity is filled with small polystyrene plastic beads, and high temperature steam is injected into the plastic beads to fuse them together. This creates a vaporizable polymeric pattern which, after removal from the tool or mold, has a configuration substantially identical to the corresponding final product casting desired. The pattern is then given a thin vapor-permeable film or coating.
The coated vaporizable pattern thus produced, together with suitable sprue and gate pieces which may also be made of a coated vaporizable polymeric material, are then buried in a sand container which may be vibrated to pack the sand about the pattern, sprue and gate pieces. As molten metal, such as grey iron or aluminum, is poured into the pattern, the polystyrene pattern vaporizes and is replaced by the molten metal. After solidification and cooling, the resulting casting is removed from the sand. In general, the lost foam process results in a casting having substantially improved dimensional accuracy, stability and surface finish over products of sand casting processes.
One attempt at making grey iron stator frames or shells by the lost foam casting process has been undertaken in the prior art by at least one foundry. The vaporizable patterns used by such foundry in making grey iron stator frames are made in two separate pieces or sections which are secured together, as by a suitable adhesive, to form a completed pattern. It is believed that this approach, however, has drawbacks in that the parting line or parting planes of the pattern (that is, the interfacing adhered surfaces) prevent the pattern from having the degree of dimensional accuracy that would be necessary to fully realize the benefits otherwise achievable with lost foam casting. Another drawback in the stator frames made by the known prior techniques using a lost foam process is that the external longitudinal cooling fins on the stator frames appear to be of equal or reduced height in comparison to the fin height of comparable size stator frames made by conventional sand casting techniques. Also, it is believed that the number of external cooling fins for a given diameter stator frame has been reduced on prior lost foam stator frames as compared to comparably sized sand cast stator frames, thereby resulting in increased circumferential spacing between fins on the lost foam stator frame.
The prior techniques and drawbacks described above appear to represent conventional thinking of persons skilled in the art. For example, when tool makers were approached by applicant to make tools or molds for making vaporizable patterns lost foam stator housing castings having external cooling fins, the conventional approach seemed to represent a bias toward making the final product configuration such that it would be easier and less costly to produce the vaporizable patterns from the tools or molds. More particularly, it is apparently believed that reducing the height and number of cooling fins on a motor stator housing makes it much easier and less expensive to make the metallic tools or molds, and to remove the vaporizable patterns from the tools. Thus, efforts to increase the number of fins (and length or height thereof) would appear to be contrary to conventional wisdom in the art, although fewer and shorter height fins would certainly seem to make it easier to remove a pattern from the tool. It thus seems that prior decisions relating to making castings to be used for motor housings have been driven by casting process artisans. However, it is believed that it would be more desirable to identify desirable criteria for the functional performance of a finished cast motor housing, and then refine or improve casting techniques to provide finished housings having such characteristics. Such characteristics are related to, among other things, the overall strength and rigidity of the fins and stator housing; the total amount of material needed to maintain a desired degree of structural integrity; heat transfer qualities of a finished housing; the amount of post casting machining that will be required; and the type of process to be used for assembling the stator and housing.
Practice of the present invention clearly contemplates increasing fin height to add strength and rigidity to the stator housing. The latter is particularly desirable to allow an increase in the rigidity of the generally annular wall which immediately surrounds the stator core, and enables the use of a thinner annular wall. The thickness of this annular wall significantly affects the heat transfer and cooling characteristics of the stator frame, and thus improved heat transfer characteristics as well as structural characteristics and material utilization are achieved.